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Rotting Apple

Each of the first four volumes provides 25 probes with easy-to-follow steps for uncovering and addressing students’ ideas by promoting learning through conceptual change instruction. Probes cover topics such as physical, life, and Earth and space science; the nature of science; and unifying themes. Each volume on page 23 provides topic-specific probes. These invaluable books include teacher materials that explain content, identify links to standards, and suggest grade-appropriate ways to present materials so students learn the concepts accurately. Teachers, professional development coordinators, and college science and preservice faculty will find these resources essential and exciting.


Rotting Apple

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The purpose of this assessment probe is to elicit students’ ideas about decay and decomposers. The probe can be used to determine whether students recognize the need for a biological agent to break down once-living material as it uses it for energy.

Related Concepts

Decay, decomposers, decomposition, microbes


The best response is Selma’s: I think small organisms use it for energy and building material. Rotting, or decay, is a natural recycling process in which the material of once-living things is broken down and released into the environment to be reassembled and used again by other organisms or incorporated into the physical environment. Natural decay does not happen without a biological agent. Dead organisms or parts of once-living organisms are broken down by biological agents, called decomposers. The material of the apple that is no longer part of the living tree is a source of food for decomposers. This food provides the energy and building material they need to live, grow, and reproduce. Decomposers include worms, insects, fungi, and bacteria. Most decay is the result of action by microorganisms (bacteria and some fungi). Decomposers are found in large numbers almost everywhere in an ecosystem, including in air, land, and water. They are prevalent in the upper layers of most soils. Fungal spores and bacteria also make up particulate matter in the air. The factors that support decay are the factors that support microbial growth, such as warmth and moisture.

Curricular and Instructional Considerations

Elementary Students

During the elementary grades, children build an understanding of biological concepts through directly observing phenomena, such as the rotting apple. Students identify factors that promote decay and observe the changes that happen to a decomposing object over time. They develop a basic understanding of decomposers and the decay process, beginning with macroscopic organisms they can readily observe, such as worms, beetles, mushrooms, and molds.

Middle School Students

In the middle school grades, students become familiar with the beneficial role of bacteria in ecosystems. They expand their understanding of decomposers and decay to include microorganisms and recognize their essential role in the decomposition process as matter recyclers. They are introduced to ideas about nutrition, matter, and energy flow, and they identify the relationships between organisms in a food web, including decomposers.

High School Students

In high school, students approach decomposition from a molecular view, including the assortment of complex biological processes involved in breaking down once-living material. They recognize the role of decomposers in cycling atoms and molecules throughout the living and nonliving components of Earth’s biosphere. They connect the natural process of biodegradation to human engineered systems that solve problems of buildup of dead material and metabolic wastes.

Administering the Probe

The answer to this probe deliberately uses the idea that small organisms use the apple for energy and building material rather than directly mentioning food, but it does not include the idea of breaking it down into simpler substances since students can choose that response to match their notion of rotting, yet not understand the decay process. When food is used in the correct response, students are apt to choose it without considering the idea of decay. To students, an apple is food and other organisms eat apples. Because the idea of a source of energy is introduced in upper elementary school, we choose to use energy in this answer rather than food. However, if you are using this probe with younger students, grades K–2, you may wish to substitute food for energy and building material and remove Felicia’s response. This probe can also be used with a prop. Show a rotted apple (preferably one that has been on the ground) and a fresh one. Other material can be substituted for the apple (e.g., pumpkins, rotted logs, dead leaves). Avoid bringing in dead and decaying animals or decayed logs with exposted fungi that can release spores.

Related Ideas in National Science Education Standards (NRC 1996)

K–4 The Characteristics of Organisms

  • Organisms have basic needs.

K–4 Organisms and Their Environments

  • All organisms cause changes in the environments where they live. Some of these changes are detrimental to the organism or to other organisms, whereas others are beneficial.

5–8 Populations and Ecosystems

  • Decomposers, primarily bacteria and fungi, are consumers that use waste materials and dead organisms for food.

9–12 The Interdependence of Organisms

  • Energy flows through ecosystems in one direction, from photosynthetic organisms to herbivores to carnivores and decomposers.

9–12 Matter, Energy, and Organization in Living Systems

  • As matter and energy flow through different levels of organization of living systems—cells, organs, organisms, communities—and between living systems and the physical environment, chemical elements are recombined in different ways.
Related Ideas in Benchmarks for Science Literacy (AAAS 1993)

K–2 Flow of Matter and Energy

  • Many materials can be recycled and used again, sometimes in different forms.

K–2 Constancy and Change

  • Things change in some ways and stay the same in some ways.

3–5 Interdependence of Life

  • Insects and various other organisms depend on dead plant and animal material for food.
  • Most microorganisms do not cause disease and many are beneficial.

3–5 Flow of Matter and Energy

  • Some source of energy is needed for all organisms to stay alive and grow.
  • Over the whole Earth, organisms are growing, dying, and decaying, and new organisms are being produced by the old ones.

6–8 Interdependence of Life

  • Two types of organisms may interact with one another in several ways: They may be in a producer-consumer, predator-prey, or parasite-host relationship. Or, one organism may scavenge or decompose another.

6–8 Flow of Matter and Energy

  • Food provides molecules that serve as fuel and building material for all organisms.

9–12 Flow of Matter and Energy

  • The amount of life any environment can support is limited by the available energy, water, oxygen, and minerals, as well as by the ability of ecosystems to recycle the residue of dead organic materials.
  • The chemical elements that make up the molecules of living things pass through food webs and are combined and recombined in different ways.

Related Research

  • Several research studies have identified children’s common notions about decay. In response to research questions related to the “disappearance” of dead animals or fruits on the surface of the soil, young children thought dead things just disappear (Driver et al. 1994).
  • Some middle school students see decay as a gradual, inevitable consequence of time without need of decomposing agents (AAAS 1993).
  • Younger students tend to think that insects break up material once it has started to rot of its own accord (Driver et al. 1994).
  • In a study of 15- to 16-year-olds, 65% used words like bacteria, fungi, and decomposers, but were not sure about their roles. Although older students tend to offer more factors to explain decay, there was little evidence that they had an understanding of how physical factors relate to the action of microbes (Driver et al. 1994, p. 65).
  • Generally, students are unaware of the role that microorganisms play in ecosystems— especially their role as agents of change, such as decomposers and recyclers (Driver et al. 1994).
  • When fifth and sixth graders were asked what makes a dead thing disappear, some of their comments included, “When it’s been dead a long time and gets real old, it breaks up and disappears,” “When the rain and wind come, the dead plant spreads out into the dirt,” “When we die they put us in a coffin and bury us, and while we’re in the coffin we dissolve” (Hogan 1994).
  • Some students say things “decay” or “rot away” without realizing that microorganisms cause the decay process as they use dead material for food (Hogan 1994).

Suggestions for Instruction and Assessment

  • In activities involving observations of changes that happen during decomposition, explicitly link the notion of a living agent that causes the changes to the changes students observe.
  • Emphasize the notion that all living things need food, including microorganisms. Food can be the sugars produced by plants and the fresh forms of plants, animals, and certain fungi we are familiar with. However, food for other organisms can be what we find offensive—rotting material and waste products. Develop the commonality that all of these sources of food serve to provide living organisms—from single-celled organisms to large plants and animals—the energy and building material for growth and repair.
  • Decay can be observed carefully by collecting dead leaves in a woodland habitat and separating them into different layers. The top layer usually consists of the whole, dead leaves. The bottom layer usually consists of the broken-down humus. In between are leaves in various stages of decay. Explicitly link observations of the changes in the leaves to the organisms responsible for the changes.
  • The primary agents of decomposition are microorganisms, and because they are not visible to the naked eye, students tend to overlook them as agents of decay. Students tend to associate microorganisms with disease and less with beneficial processes like decay of dead plants and animals. Explicitly develop an appreciation of the role of microorganisms as decomposers and recyclers.
  • With young children, be aware that focusing on and investigating the role of one organism, such as worms, in breaking down material may lead students to a narrow view of what kinds of organisms are considered to be decomposers.
  • Explicitly ask students what makes things rot and draw a picture or create storyboards of rotting, explaining what happens in each stage. Look for evidence of physical processes, such as the action of heat and water versus biological processes.
  • Elicit students’ conception of what food is. Ask them if a rotten apple or a decaying animal is food and probe further for their ideas.
  • Encourage students to develop operational definitions of decay and decomposition before introducing the scientific terminology.
  • Use safe practices when bringing in decaying plant material for students to observe. Some students have allergic reactions to mold spores. It is best to place rotting material in a closed container for student observations.
  • Make connections to human-designed systems that take advantage of decomposition, such as composting.

Related NSTA Science Store Publications and Journal Articles

  • Ashbrook, P. 2004. Teaching through trade books: Meet the decomposers. Science and Children (Summer): 14–16.
  • Driver, R., A. Squires, P. Rushworth, and V. WoodRobinson. 1994. Making sense of secondary science: Research into children’s ideas. London and New York: RoutledgeFalmer.
  • Keeley, P. 2005. Science curriculum topic study: Bridging the gap between standards and practice. Thousand Oaks, CA: Corwin Press.
  • Trautman, N., and Environmental Inquiry Team. 2003. Decay and renewal. Arlington, VA: NSTA Press.
  • Related Curriculum Topic Study Guide (Keeley 2005) “Decomposers and Decay”

American Association for the Advancement of Science (AAAS). 1993. Benchmarks for science literacy. New York: Oxford University Press.

Driver, R., A. Squires, P. Rushworth, and V. Wood-Robinson. 1994. Making sense of secondary science: Research into children’s ideas. London and New York: RoutledgeFalmer.

Hogan, K. 1994. Eco-inquiry: A guide to ecological learning experiences for the upper elementary/ middle grades. Millbrook, NY: Institute of Ecosystem Studies.

Keeley, P. 2005. Science curriculum topic study: Bridging the gap between standards and practice. Thousand Oaks, CA: Corwin Press.

National Research Council (NRC). 1996. National science education standards. Washington, DC: National Academy Press.

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